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I have a 742 engine that given to me and a gear is stripped in the mechanism. Is there any way to repair or replace the mechanism in this 1950's engine?
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From what I've heard, a Hafner or Marx engine should be usable.
I replaced on with a Lionel can motor mechanism that had blue wheels. All it took was to fabricate a front motor mount.
Unique Arts are one of the more difficult windup trains to repair, for two reasons: 1) Very few around, hard to find parts/donor motors and 2) They were never intended to be disassembled, to the point of the motor being riveted in the shell. But, just because it is difficult, doesn't mean it can't be done.
Looking at my UA 742, it appears that it has brass pinion gears that are probably 48 pitch. It may be possible to get a workable replacement from Stock Drive Products, or one might be sourced from a more common windup motor such as Marx. The rivets in the front motor mount will need to be drilled out of the shell. These can either be re-riveted, or use machine screws to replace them. The motor looks like it is swaged together similar to a Marx motor, so it shouldn't be too hard to get it apart. Unless you can find a donor motor for the gears, it will take some machine work to retrofit a replacement pinion. I'm assuming that a pinion gear stripped; it is very unusual for a big stamped steel gear to strip (but not impossible, of course). Depending on what gear is stripped, it may or may not be possible to get to it without pulling a drive wheel.
If you want to replace the motor with a more common brand, you will need to fabricate some motor mounts. The UA motor is larger than a Marx or Hafner windup motor, so it won't be a drop-in replacement. However, fabricating motor mounts will probably be easier for most people than repairing the motor.
If I can be of assistance, feel free to email me (email address is in my profile). If you decide to replace the motor, I would be happy to buy the broken motor... 
